Thermal management in semiconductor devices and circuits is a critical design element in any manufacturable and cost-effective electronic and optoelectronic product, such as light generation and electrical signal amplification. The goal of efficient thermal design is to lower the operating temperature of such electronic or optoelectronic devices while maximizing performance (power and speed) and reliability. Examples of such devices are microwave transistors, light-emitting diodes and semiconductor lasers. Depending on the frequency of operation and power requirements, these devices have been conventionally made on silicon, gallium arsenide (GaAs), indium phosphide (InP), and in recent years gallium nitride (GaN), aluminum nitride (AlN) and other wide-gap semiconductors. Gallium nitride material systems in particular give rise to microwave transistors with high-electron mobility (necessary for high-speed operation), high breakdown voltage (necessary for high power), and thermal conductivity that is greater than GaAs, InP, or silicon, and thus advantageous for use in high power applications. GaN is also used in manufacturing of blue and ultraviolet lasers and light-emitting diodes. In spite of the high-temperature performance, GaN electronic and optoelectronic devices are limited in performance due to relatively low thermal resistance of the substrates commonly used for growth of GaN. This deficiency is most pronounced in high-power microwave and millimeter-wave transistors and amplifiers where reduced cooling requirements and longer device life, both benefiting from lower junction temperature, are in critical demand. Similar need is exhibited in high-power blue and ultraviolet lasers where a several-micrometer-wide laser cavity stripe dissipates power into the chip though low thermal conductivity materials.
It is well known that when considering isotropic behaviors diamond is the most thermally conductive substance known to man at room temperature. For this reason, the semiconductor industry has been employing diamond heat-sinks and heat spreaders for improved thermal management since the commercialization of synthetic diamond by chemical-vapor deposition in the 1980s. The objective of optimal heat management is to bring the diamond heat-spreader or diamond layers into close proximity to the heat source in the electronic or optoelectronic devices. This means building devices on thin chips and mounted on diamond heat-spreaders, coating devices with diamond layers, or transferring device epilayers (epitaxially grown semiconductor layers) onto diamond.
GaN-on-diamond technology and resulting devices (described in U.S. Pat. No. 7,595,507) involve structures which feature GaN epilayers less than a micron from a CVD diamond substrate. This technology enables bringing together the best heat conductor (diamond) together with electronic and optoelectronic devices based on gallium-nitride (GaN) and GaN-related compounds while minimizing any thermal barrier associated with, for example, more common semiconductor-solder-diamond attachment schemes. Due to GaN's inherent high critical electrical field and wide bandgap, GaN devices are preferred for high power electrical and optoelectronic applications, such as, high power RF transistors and amplifiers, power management devices (Schottky diodes and switching transistors), as well as, high power blue and ultraviolet lasers or light-emitting diodes.
GaN is presently grown on several different substrates: sapphire, silicon, silicon carbide, aluminum nitride, single-crystal diamond, and GaN substrates. With the exception of GaN substrates, all other materials have lattice constants that differ from that of GaN and AlGaN. Natural diamond is an excellent thermal conductor, but has not been available for these applications due to its available area, reduced thermal properties over high purity synthetic diamond, and cost. Presently, synthetic diamond is being manufactured with various degrees of crystallinity. Polycrystalline diamond deposited by chemical-vapor deposition (CVD) is suitable for use in the semiconductor industry as its thermal conductivity is close to that of single crystal diamond, it can provide electrical isolation, has low dielectric losses, and can be made transparent. CVD diamond substrates for semiconductor industry can be formed as round wafers with standard diameters. Diamond wafers are manufactured by chemical vapor deposition by one of three main methods: plasma enhanced diamond CVD where the energy to dissociate the reactants comes from a microwave source, thermally assisted diamond CVD where the energy for dissociating gasses comes from a hot filament, and plasma torch where ions are accelerated using a high DC voltage. In these processes, synthetic diamond is grown on top of non-diamond substrates, such as, silicon, silicon nitride, silicon carbide and different metals.
The CVD diamond growth process is carried out in a vacuum chamber within which a substrate is provided on top of which diamond is to be grown. The substrate is exposed to the energy source needed to dissociate molecules of precursor gases needed to form diamond on the surface of the substrate. The precursor gases needed in the chemical vapor deposition of diamond are a source of carbon diluted in hydrogen (H2). Typical carbon-carrying gases are methane (CH4), ethane (C2H6), carbon monoxide (CO), and acetylene (C2H2), with methane (CH4) being the most commonly used. The gas combination needed for efficient diamond deposition contains a small (several percent) composition of the carbon-carrying gas in hydrogen, and the reaction can be further assisted with addition of oxygen or oxygen precursors such as CO or CO2. A most common parameter specifying the gas-flow recipe is given in terms of the molar ratio of carbon carrying gas flow and hydrogen gas flow. For example, in terms of percentage [CH4]/[H2] where [CH4] and [H2] are molar flow rates typically measured in standard cubic centimeters per minute (sccm). Typical substrate temperatures during the deposition process are between 550° C. and 1200° C., and deposition rates are usually measured in micrometers (μm) per hour.
Growth of synthetic diamond on non-diamond substrates includes a surface preparation phase and a nucleation phase in which conditions are adjusted to enhance the growth of diamond crystals on the host (non-diamond) substrate. This is most commonly done by seeding (linked also to substrate scratching) the surface with diamond powder in a controlled and repeatable manner. During the growth phase, the grain size of synthetic diamond increases and as a result synthetic diamond films are inherently rough after deposition. The nucleation of diamond generally starts with very small diamond domains embedded in non-diamond matrix which has poor thermal conductivity in the near-substrate regions. Various types of seeding have been discussed in the prior art including mechanical, ultrasonic and mega-sonic seeding of nucleation layers on various substrates and wafers.
The increasing high power density in GaN-based HEMTs (high electron mobility transistors) makes thermal management critically important. CVD polycrystalline diamond of high thermal conductivity offers superior heat removal capability near the device junction compared to state-of-the-art SiC substrates. The latest GaN-on-diamond HEMTs have demonstrated excellent device characteristics [D. C. Dumka et al., IEEE Electron Lett. 49(20), 1298 (2013)] and are scalable to 4-inch wafers [D. Francis et al., Diamond Rel. Mater. 19(2-3), 229 (2010)]. This GaN-on-diamond technology starts with a MOCVD-grown AlGaN/GaN epilayer on silicon or silicon carbide, and involves depositions of a thin dielectric seeding layer which may be amorphous or polycrystalline (e.g. silicon carbide, silicon, silicon nitride, aluminium nitride, magnesium oxide, boron nitride, or beryllium oxide) and CVD diamond on the exposed GaN, following the removal of the native GaN growth substrate and transition layers [D. C. Dumka et al., IEEE Electron Lett. 49(20), 1298 (2013); D. Francis et al., Diamond Rel. Mater. 19(2-3), 229 (2010)]. The dielectric seeding layer serves as both a nucleation layer for the diamond material and a protective layer for the GaN during diamond growth. As such, the dielectric seeding layer must be sufficiently thick to fulfill these functions. However, the dielectric interlayer and the initial nucleation layer of diamond growth result in an effective thermal boundary resistance (TBReff) at the GaN/diamond interface, which is a major thermal barrier that limits the full thermal benefit of diamond [J. W. Pomeroy et al., Appl. Phys. Lett. 104(8), 083513 (2014)].
To date, direct growth of diamond on GaN has been problematic. This has primarily been due to reaction of atomic hydrogen with exposed GaN and the subsequent degradation and reduction of the GaN substrate. The typical method for circumventing the problem known to practitioners in the art has been to grow a dielectric interlayer on top of the GaN which serves as both a protective layer for GaN and a nucleation layer for diamond as described above. While this approach has been successful in protecting the GaN layer, it has introduced multiple thermal boundaries that negatively impact the total thermal resistance and full benefits of a highly conductive substrate. In addition, the requirement for a dielectric interlayer between the GaN and the diamond introduces additional surface preparation and deposition steps into the fabrication process which increase the complexity and expense of the fabrication process.
A significant challenge in achieving intimate integration of diamond with GaN, lies in balancing the reduction of thermal boundary resistance (TBR) due to various layers at the interface of GaN and diamond, achieving the right level of seeding for robust adhesion to the nucleating layer(s), and providing sufficient protection for the underlying GaN when depositing CVD diamond thereon so as to not adversely affect the electronic properties of the GaN epilayer structure. The present inventors have studied the effect of the dielectric interlayer thickness on the effective thermal boundary resistance (TBReff) at the GaN/diamond interface. The present inventors have previously found that a dielectric interlayer of at least 35 nm thickness is required to protect a GaN substrate during CVD diamond growth thereon. However, this results in a lower limit to the effective thermal boundary resistance between the GaN and diamond of about 25 m2 K/GW.